Plasma is generally considered to be a fourth state of matter, the others being solid, liquid and gas states. In the plasma state the elementary constituents of a substance are substantially in an ionized form rendering them useful for many applications due to, inter alia, their enhanced reactivity, energy, and suitability for the formation of directed beams.
Plasma generators are routinely used in the manufacture of electronic components, integrated circuits, and medical equipment, and in the operation of a variety of goods and machines. For example, plasma is extensively used to deposit layers of a desired substance, for instance following a chemical reaction or sputtering from a source, to etch material with high precision, and to sterilize objects by the free radicals in the plasma or induced by the plasma or to modify surface properties of materials.
Plasma generators based on radio frequency (“RF”) power supplies are often used in experimental and industrial settings since they provide a ready plasma source, and are often portable and easy to relocate. Such plasma is generated by coupling the RF radiation to a gas, typically at reduced pressure (and density), causing the gas to ionize. In any RF plasma production system, the plasma represents a variable load at the antenna terminals as the process conditions changes. Among other process control factors, changes in working gas and pressure affect the amount of loading seen at the antenna terminals. In addition, the amplitude of the RF drive waveform itself affects the plasma temperature and density, which in turn also affects the antenna loading. Thus the antenna/plasma combination represents a non-constant and nonlinear load for the RF power source to drive.
A typical RF source has a 50 ohm output impedance, and requires a load that presents a matching 50 ohm impedance in order for the RF source to couple to the load most efficiently. Because of the often unpredictable changes in the plasma self inductance, effective resistance, and mutual inductance to the antenna, provision for impedance matching is made by retuning some circuit elements and possibly the plasma to obtain satisfactory energy transfer from the RF source to the generated plasma. To achieve this, an adjustable impedance matching network, or “matching box” is typically used to compensate for the variation in load impedance due to changes in plasma conditions. The matching box typically contains two independent tunable components, one that adjusts the series impedance and the other that adjusts the shunt impedance. These components must be adjusted in tandem with each other in order to achieve the optimum power transfer to the plasma. Not surprisingly, accurate tuning of these components is often a difficult process. Typically, retuning requires manual/mechanical operations/actuators to adjust one or more component values and generally sophisticated feedback circuitry for the rather limited degree of automation possible.
It is well known that the application of a sufficiently large electric field to a gas separates electrons from the positively charged nuclei within the gas atoms, thus ionizing the gas and forming the electrically conductive fluid-like substance known as plasma. Coupling radio frequency electric and magnetic fields to the gas generates, via an antenna, induces currents within this ionized gas. This, in turn, causes the gas to further ionize, and thereby increasing its electrical conductivity, which then increases the efficiency with which the antenna fields couple to the charged particles within the gas. This leads to an increase in the induced currents, resulting in the electrical breakdown and substantial ionization of the gas by various mechanisms. The effectiveness of the RF coupling is dependent upon the particular RF fields and/or waves that are used. Some types of waves that are suitable for the efficient production of large volumes of plasma are described next.
Whistler waves are right-hand-circularly-polarized electromagnetic waves (sometimes referred to as R-waves) that can propagate in an infinite plasma that is immersed in a static magnetic field Bo. If these waves are generated in a finite plasma, such as a cylinder, the existence of boundary conditions—i.e. the fact that the system is not infinite—cause a left-hand-circularly-polarized mode (L-wave) to exist simultaneously, together with an electrostatic contribution to the total wave field. These “bounded Whistler” are known as Helicon waves. See Boswell, R. W., Plasma Phys. 26, 1147 (1981). Their interesting and useful qualities include: (1) production and sustenance of a relatively high-density plasma with an efficiency greater than that of other RF plasma production techniques, (2) plasma densities of up to Np˜1014 particles per cubic centimeter in relatively small devices with only a few kW of RF input power, (3) stable and relatively quiescent plasmas in most cases, (4) high degree of plasma uniformity, and (5) plasma production over a wide pressure range, from a fraction of a mTorr to many tens of mTorr. Significant plasma enhancement associated with helicon mode excitation is observed at relatively low Bo-fields, which are easily and economically produced using inexpensive components.
Significant plasma density (Np) enhancement and uniformity may be achieved by excitation of a low-field m=+1 helicon R-wave in a relatively compact chamber with Bo<150 G. This may be achieved, for instance, through the use of an antenna whose field pattern resembles, and thus couples to, one or more helicon modes that occupy the same volume as the antenna field. The appropriate set of combined conditions include the applied magnetic field Bo, RF frequency (FRF),), the density Np itself, and physical dimensions.
Some antenna designs for coupling RF power to a plasma are disclosed by U.S. Pat. Nos. 4,792,732, 6,264,812 and 6,304,036. However, these designs are relatively complex often requiring custom components that increase the cost of system acquisition and maintenance. Moreover, not all of the designs are suitable for efficient generation of the helicon mode, which is a preferred mode disclosed herein.
RF power sources typically receive an external RF signal as input or include an RF signal generating circuit. In many processing applications, this RF signal is at a frequency of 13.56 MHz, although this invention is not limited to operation at this frequency. This signal is amplified by a power output stage and then coupled via an antenna to a gas/plasma in a plasma generator for the production of plasma. Amplifiers are conventionally divided into various classes based on their performance characteristics such as efficiency, linearity, amplification, impedance, and the like, and intended applications. In power amplification, an important concern is the amount of power wasted as heat, since heat sinks must be provided to dissipate the heat and, in turn, increase the size of devices using an inefficient amplifier. A classification of interest is the output impedance presented by an amplifier since it sets inherent limitations on the power wasted by an amplifier.
Typical RF amplifiers are designed to present a standard output impedance of 50 Ohms. Since, the voltage across and current through the output terminals of such an amplifier are both non-zero, their product provides an estimate of the power dissipated by the amplifier. In contrast to such amplifiers, a switch presents two states: it is either ON, corresponding to a short circuit, i.e., low impedance, or OFF, corresponding to an open circuit, i.e., infinite (or at least a vary large) impedance. In switched mode amplifiers, the amplifier element acts as a switch under the control of the signal to be amplified. By suitably shaping the signals, for instance with a matching load network, it is possible to introduce a phase difference between the current and the voltage such that they are out of phase to minimize the power dissipation in the switch element. In other words, if the current is high, the voltage is low or even zero and vice versa. U.S. Pat. Nos. 3,919,656 and 5,187,580 disclose various voltage/current relationships for reducing or even minimizing the power dissipated in a switched mode amplifier.
U.S. Pat. No. 5,747,935 discloses switched mode RF amplifiers and matching load networks in which the impedance presented at the desired frequency is high while harmonics of the fundamental are short circuited to better stabilize the RF power source in view of plasma impedance variations. These matching networks add to the complexity for operation with a switched mode power supply rather than eliminate the dynamic matching network.
The problems faced in efficient plasma generator design include the need for a low maintenance and easily configured antenna, the elimination of expensive and limited matching networks to couple the RF power source to the non-linear dynamic impedance presented by a plasma, and the need for RF power sources that can be efficiently modulated.